ABSTRACT This grant will investigate the regulation of apoptosis during cerebellar development and in medulloblastoma, in order to gain new information on the pathogenesis of microcephaly and on brain tumor treatment. Medulloblastoma, the most common malignant brain tumor in children, arises from cerebellar progenitors that proliferate in the postnatal brain. We propose that cerebellar progenitors and medulloblastoma cells share a specialized mechanism of apoptosis regulation that makes the developing brain susceptible to growth failure and also makes medulloblastoma vulnerable to radiation and chemotherapy. Directly targeting this apoptosis mechanism may be a new way to treat medulloblastoma with greater efficacy and reduced toxicity. We have shown that neural progenitors and medulloblastoma cells maintain a ?primed-for- death? state, in which the pro-apoptotic protein BAX is constitutively activated. These cells depend on anti-apoptotic proteins to prevent BAX from inducing spontaneous apoptosis. In our preliminary studies, we deleted the anti-apoptotic protein Bcl-xL in cerebellar progenitors to determine if BCL-xL is required for cerebellar development, and if targeting BCL-xL can impair medulloblastoma growth. We found that Bcl-xL deletion caused cerebellar progenitors to die as they exited the cell cycle. This effect blocked cerebellar growth, but surprisingly did not fully prevent medulloblastomas from growing in medulloblastoma-prone mice. Also surprising was that Bcl-xL-deleted progenitors showed increased proliferation. Based on these findings, in Aim 1 we propose to identify additional apoptosis regulators that work with BCL-xL to govern the survival of cerebellar progenitors and medulloblastoma cells. In Aim 2, we will test the hypothesis that Bcl-xL-deleted progenitors have increased proliferation because BCL-xL is required for the process of differentiation. BCL-xL has been implicated in mitochondrial function, and we have previously shown that oxidative metabolism plays an essential role in the differentiation of cerebellar progenitors. We will block apoptosis by Caspase inhibition and then determine whether BCL-xL is required for the transition from aerobic glycolysis to oxidative phosphorylation during progenitor differentiation. In Aim 3, we will use a primary mouse tumor model to examine whether inducing differentiation in medulloblastoma increases the anti-tumor effect of Bcl-xL deletion. We will also test a brain-permeant, nanoparticle-delivered BCL-xL inhibitor that we have developed as a potential medulloblastoma therapy. These Aims will show how BCL-xL regulates progenitor survival during brain growth, and test the hypothesis that BCL-xL can be targeted to improve medulloblastoma therapy.